3GPP (3rd Generation Partnership Project), which is a project aiming to standardize a mobile communication system, plans to introduce a dual connectivity scheme (Dual connectivity) in Release 12 and onward (see Non Patent Document 1). In the dual connectivity scheme, a user terminal simultaneously establishes a connection with a plurality of base stations. A user terminal is allocated, from each base station, with radio resources, and thus, it is possible to expect an improvement in throughput.
In the dual connectivity scheme, of the plurality of base stations that establish a connection with the user terminal, only one base station (hereinafter, called “master base station”) establishes an RRC connection with the user terminal. On the other hand, of the plurality of base stations, another base station (hereinafter, called “secondary base station”) provides additional radio resources to the user terminal without establishing an RRC connection with the user terminal. It is noted that the dual connectivity scheme may also be called an inter-base station carrier aggregation (inter-eNB CA).